There have been numerous attempts to express selected genes of interest using a variety of different cell lines. Conventional established mammalian cell lines, for instance, have been widely utilized in light of their capacity to grow quickly to high cell densities and in suspension, both desirable characteristics for the production of biological products. Examples of commonly used cell lines in this regard include the BHK, CHO, arid COS cell lines. Often however, the growth of established cell lines such as these cannot be sufficiently controlled. This is a significant disadvantage in situations in which a high cell density has been achieved, and the primary purpose becomes collection of the biological product rather than generation of increasing cell numbers. In addition, established cell lines almost without exception exhibit an abnormal karyotype. These cell lines are often either tumorigenic in vivo, or give rise to tumorigenic cells upon further proliferation and extended culture. Tumorigenicity is a significant concern when selecting a cell line from which biological products will be isolated.
An additional disadvantage of conventional established cell lines is that they are isolated and propagated in culture media containing a serum supplement. Further, these cell lines often require a serum supplement for continued growth (Barnes and Sato, Cell 22:649-655, 1980). This requirement for serum creates a source of additional expense, problems relating to quality control and reproducibility of the serum, and problems in isolating desired biological substances.
In contrast to established cell lines, normal cell lines in early passages exhibit a predominantly diploid karyotype and are non-tumorigenic. However, these cells, known as primary cells, characteristically undergo alterations upon multiple passages. Consequently, the culture eventually degenerates or undergoes multiple genetic and phenotypic changes, often resulting in the development of tumorigenic cells, making further passage of primary cells undesirable for the production of biological products.
While there have been attempts to utilize serum-free media to generate cell lines capable of continuous growth without subsequent degeneration or chromosomal aberration, these efforts have been only partially successful. Consequently, there is a need in the art for a cell line capable of indefinite growth under serum-free conditions, the cell line further being capable of expressing exogenously introduced genes. In addition, the cell line should be capable of growth to high density in suspension, the growth of the cell line also being subject to selective control. The present invention fulfills this need and further provides other related advantages.